LITHIUM-TO-CALCIUM RATIOS IN MODERN, CENOZOIC, AND PALEOZOIC ARTICULATE BRACHIOPOD SHELLS

Li/Ca ratios in modern brachiopod shells generally correlate inversely with growth temperature, ranging from similar to 20 mu mol/mol at 30 degrees C to similar to 50 mu mol/mol at 0 degrees C with no apparent interspecific offsets. Causes of the temperature effect on Li/Ca ratios are not yet understood. Cenozoic brachiopod Li/Ca ratios average similar to 30 mu mol/mol, similar to the average observed in modern brachiopods. Relatively constant Li/Ca ratios for Eocene to Pleistocene nonluminescent brachiopod shells, consistent with previous observations of Cenozoic planktonic foraminifera, support the conclusion of little variation in Cenozoic seawater Li/Ca. Nonluminescent portions of Permian and Carboniferous brachiopods have Li/Ca ratios substantially lower (generally <10 mu mol/mol) than modern, Cenozoic, or Devonian samples. Mass balance considerations, constrained by delta O-18 of brachiopods, suggest that low Li concentrations in Permo-Carboniferous seawater could be the result of a lower flux of dissolved Li from the continents and/or a higher flux of Li from seawater to clastic marine sediments. Nonluminescent Devonian brachiopods from a single hand specimen have Li/Ca ratios around 70% of the modern average. These Li/Ca ratios can be explained by either somewhat higher temperature with constant seawater Li/Ca, somewhat lower seawater Li/Ca at constant temperature, or a combination of slightly elevated temperature and slightly lower seawater Li/Ca.